The invention relates to a method and to an apparatus for determining the load bearing capacity of a road surface, in accordance with the preamble of the independent method claim and the independent apparatus claim respectively.
Thus, it is well known to determine the bearing capacity of a road surface by measuring its elastic resiliency, for instance by driving a vehicle along the road and loading the road surface with a specific force via a small loading area that moves with the vehicle, for instance via a vehicle rear wheel, wherewith at least two measurements are taken at each chosen measuring position on the road surface, said measurements being taken at different distances in the direction of vehicle movement. For instance, one measurement can be taken when the measuring area is located closely adjacent to the loading area, and when the measuring area is located relatively far from the loading area and the road surface is still unaffected by the load or has sprung back resiliently when the load is removed from said area. The measuring operation involves detecting the vertical distance between the measuring location and a vehicle mounted reference. The measurement is carried out suitably with the aid of some contactless process. The bearing capacity of the road surface can be determined, by measuring the elastic deformation of said surface in the area around the loading area.
U.S. Pat. Nos. 4,781,058, 4,571,695, 4,456,829, DE-A1-36 43 470 and WO 96/31655 are prior publications that teach the present state of techniques.
One problem with present technology is that the road surface is relatively uneven and that it is difficult to carry out the second measuring process at the same place on the road surface as that on which the first measurement was taken. It is, of course, possible to try to read the surface pattern of the road surface at the measuring location in conjunction with the first measuring process, and to try to identify this pattern in conjunction with the second measuring process and to carry out said second measuring process at the correct location. The difficulty here, however, is that the smoother the road surface, the more difficult it is to recognise the pattern.
The choice of a relatively wide measuring area carries with it a serious risk of losing valuable information. On the other hand, if it is elected to carry out the first measuring process in a very small measuring area, there is a serious danger that the process will result in erroneous measurements unless the second measuring process is carried out at precisely the same position as the first measuring process, due to the fact that the road surface will often have irregularities in the order of a millimetre whereas the elastic depression of the road surface often lies in the order of up to some millimetres.
The object of the present invention is to provide a solution to this problem. The aim of the invention is thus to provide a technique that will enable the measuring position to be re-found easily in conjunction with the second measuring process, and to provide a smooth measuring surface in said measuring location so as to avoid the drawback associated with irregularities in or the unevenness"" of the road surface.
This object is achieved by means of the invention as defined in the accompanying independent method claim.
The object is also achieved by means of the apparatus defined in the accompanying independent apparatus claim.
Other embodiments of the invention are set forth in the accompanying dependent claims.
Basically, the invention entails placing a measuring body out on the road surface. Both the first and the second measuring processes are carried out on this measuring body. The measuring body is chosen so that it can be easily detected, so that both measuring processes can be carried out at one and the same location. The measuring body is also conveniently designed to provide an essentially smooth measuring surface.
In one embodiment of the invention, there is formed a ball of relatively soft asphalt which is projected from the vehicle onto the road surface such that the ball sticks firmly to said surface while being flattened to some extent by its impact therewith. The ball is preferably projected so that it strikes the road surface at a position that is swept by two vehicle-mounted distance detectors or sensors. Naturally, the ball may alternatively be thrown to land on the road surface at a predetermined distance from the nearest measuring apparatus, so that the time distance between projection of the ball and the time at which the ball lands on the measuring area of the measuring apparatus can be determined on the basis of the current speed of the vehicle and on other factors.
The time point at which the measuring area of the second measuring apparatus covers the measuring body can also be calculated in a corresponding manner, on the basis of knowledge of the distance between the measuring areas of the measuring apparatus over the road surface and the vehicle speed. This enables the measuring processes to be limited solely to those times at which the measuring body can be expected to be located in the measuring area of the measuring apparatus.
The physical construction of the measuring apparatus is of subordinate interest, and measuring apparatus that can be used in this respect are described in the aforesaid prior publications.